1. Field of the Invention
The invention relates generally to semiconductor lasers and related optoelectric devices. In particular, the invention relates to the formation and passivation of the facets of edge-emitting optical devices.
2. Technical Background
Semiconductor lasers have found increasing use as a source of narrow bandwidth, high-power light. In particular, semiconductor lasers based upon III-V semiconductor compounds are widely used for high-speed optical recording, high-speed printing, single-mode and multi-mode data networks, long-distance networks, local-area networks, submarine cable transmission, free-space communication, doppler optical radar, optical signal processing, high-speed optical microwave sources, pump sources for other solid-state lasers, fiber amplifiers, and medical applications including both imaging and cauterization. Many of these applications require very high optical power levels, for example, above 100 mW and sometimes several watts of optical power. One particular example of a high-power application is a diode laser emitting at 980 nm for optically pumping an erbium-doped (Ber3+) fiber amplifier (EDNA).
One material combination of III-V semiconductors that is quite useful for near-infrared lasers includes layers of Al1xe2x88x92xGaxAs, GaAs, and possibly other related materials grown on Galas wafers. The most prevalent configuration is an edge-emitting laser in which a substantial number of AlGaAs/GaAs layers are grown on the Galas substrate to define the diode and vertical laser structure with some kind of lateral definition of an overlying ridge or an underlying layer to define the horizontal laser structure and lateral current confinement to the laser area. Thereafter, the wafer is cleaved at parallel natural (110) cleavage planes, between which the laser waveguide runs. The two cleavage planes form the two end facets of the laser. A high reflectivity coating is usually applied to the back facet, but the inherent reflection at the front facet is sometimes sufficient to provide the required amount of reflectance and transmittance at the front facet, which both defines the front side of the optical cavity but through which some radiation is transmitted as the laser output. Indeed, it is often found necessary to coat the front facet with an anti-reflection coating (ARC) to reduce its reflectance back into the laser cavity.
Prior to the deposition of any reflectivity coating, the facets are typically coated with a passivation layer to prevent the passage of oxygen which would oxidize the laser material. Passivation layers of Si, amorphous hydrogenated silicon (a-Si:H), Ga, Sb, ZnS, and ZnSe have been promoted. Aluminum-containing layers in the diode structure are especially prone to fast oxidation. The oxidation has been shown to introduce electronic states at the semiconductor surface. It is believed that these oxidation-induced states are responsible for the irreversible destruction of the facets, especially for high-power optical outputs, by the so called catastrophic optical damage (COD) process.
For mass production of such laser diodes, it is desired to cleave the wafers in air. However, an optical chip cleaved in air oxidizes in the period before the passivation layer is deposited. Cleaving in vacuum and then depositing the passivation layer without breaking vacuum is possible but not cost effective. As a result, in commercial production, the facets are cleaned and the oxide removed prior to coating the passivation layer in a system allowing both cleaning and coating to be performed without breaking vacuum. Cleaning by ion beam irradiation or pulsed UV laser irradiation has proved relatively successful.
The invention includes a method of passivating facets of edge-emitting semiconductor lasers and similar devices to produce a device having an amorphous hydrogenated silicon (a-Si:H) passivating layer applied to an oxide-free facet. The invention is particularly useful for a high-power laser or optical amplifier formed with a multi-mode waveguide.
After the onto-electronic chip has been cleaved along its two facets, possibly while exposed to an oxidizing air environment, the chip is placed in a vacuum controlled system capable of performing several steps without breaking vacuum. The facets are cleaned with either an ion beam or a plasma of hydrogen, possibly with the addition of argon or xenon. Then, the amorphous hydrogenated silicon passivation layer is deposited, preferably by sputtering a silicon target in the presence of a hydrogen plasma. Exemplary thicknesses of the passivation layers are 10 to 50 nm.
Preferably, the cleaning and depositing steps are performed in the same chamber without moving the substrate being coated, and the hydrogen plasma is advantageously maintained during the cleaning and coating sequence.
Optical layers, such as interference mirrors and anti-reflection coatings, are deposited over the a-Si:H passivation layers. Preferably, the optical layers are also deposited in the same system without breaking vacuum.